2 Beginning Student with List Abbreviations

On this page:

2.1 Pre-defined Variables

2.2 Template Variables

2.3 Syntaxes for Beginning Student with List Abbreviations

unquote-splicing

2.4 Common Syntaxes

check-satisfied

check-member-of

2.5 Pre-defined Functions

2.6 Numbers: Integers, Rationals, Reals, Complex, Exacts, Inexacts

current-seconds

make-rectangular

number->string-digits

boolean->string

2.9 Lists

2.10 Posns

2.11 Characters

char-alphabetic?

char-lower-case?

char-upper-case?

char-whitespace?

string-alphabetic?

string-contains-ci?

string-contains?

string-downcase

string-lower-case?

string-numeric?

string-upper-case?

string-whitespace?

2.13 Images

2.14 Misc

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2 Beginning Student with List Abbreviations

The grammar notation uses the notation X ... (bold dots) to indicate that X may occur an arbitrary number of times (zero, one, or more). Separately, the grammar also defines ... as an identifier to be used in templates.

See How to Design Programs/2e, Intermezzo 1 for an explanation of the Beginning Student Language.

See How to Design Programs/2e, Intermezzo 2 for an explanation of quoted lists.

program		=		def-or-expr ...

def-or-expr		=		definition
		\|		expr
		\|		test-case
		\|		library-require

definition		=		(define (name variable variable ...) expr)
		\|		(define name expr)
		\|		(define name (lambda (variable variable ...) expr))
		\|		(define-struct name (name ...))

expr		=		(name expr expr ...)
		\|		(prim-op expr ...)
		\|		(cond [expr expr] ... [expr expr])
		\|		(cond [expr expr] ... [else expr])
		\|		(if expr expr expr)
		\|		(and expr expr expr ...)
		\|		(or expr expr expr ...)
		\|		name
		\|		’quoted
		\|		‘quasiquoted
		\|		’()
		\|		number
		\|		boolean
		\|		string
		\|		character

quoted		=		name
		\|		number
		\|		string
		\|		character
		\|		(quoted ...)
		\|		’quoted
		\|		‘quoted
		\|		,quoted
		\|		,@quoted

quasiquoted		=		name
		\|		number
		\|		string
		\|		character
		\|		(quasiquoted ...)
		\|		’quasiquoted
		\|		‘quasiquoted
		\|		,expr
		\|		,@expr

test-case		=		(check-expect expr expr)
		\|		(check-random expr expr)
		\|		(check-within expr expr expr)
		\|		(check-member-of expr expr ...)
		\|		(check-range expr expr expr)
		\|		(check-satisfied expr name)
		\|		(check-error expr expr)
		\|		(check-error expr)

library-require		=		(require string)
		\|		(require (lib string string ...))
		\|		(require (planet string package))

package		=		(string string number number)

A name or a variable is a sequence of characters not including a space or one of the following:

" , ' ` ( ) [ ] { } | ; #

A number is a number such as 123, 3/2, or 5.5.

A boolean is one of: #true or #false. Alternative spellings for the #true constant are #t, true, and #T. Similarly, #f, false, or #F are also recognized as #false.

A symbol is a quote character followed by a name. A symbol is a value, just like 42, '(), or #false.

A string is a sequence of characters enclosed by a pair of ". Unlike symbols, strings may be split into characters and manipulated by a variety of functions. For example, "abcdef", "This is a string", and "This is a string with \" inside" are all strings.

A character begins with #\ and has the name of the character. For example, #\a, #\b, and #\space are characters.

In function calls, the function appearing immediately after the open parenthesis can be any functions defined with define or define-struct, or any one of the pre-defined functions.

2.1 Pre-defined Variables

value
empty : empty?

The empty list.

value
true : boolean?

The #true value.

value
false : boolean?

The #false value.

2.2 Template Variables

syntax
..

A placeholder for indicating that a definition is a template.

syntax
...

A placeholder for indicating that a definition is a template.

syntax
....

A placeholder for indicating that a definition is a template.

syntax
.....

A placeholder for indicating that a definition is a template.

syntax
......

A placeholder for indicating that a definition is a template.

2.3 Syntaxes for Beginning Student with List Abbreviations

syntax
’name
syntax
’part
syntax
(quote name)
syntax
(quote part)

A quoted name is a symbol. A quoted part is an abbreviation for a nested lists.

Normally, this quotation is written with a ', like '(apple banana), but it can also be written with quote, like (quote (apple banana)).

syntax
‘name
syntax
‘part
syntax
(quasiquote name)
syntax
(quasiquote part)

Like quote, but also allows escaping to expression “unquotes.”

Normally, quasi-quotations are written with a backquote, `, like `(apple ,(+ 1 2)), but they can also be written with quasiquote, like (quasiquote (apple ,(+ 1 2))).

syntax
,expression
syntax
(unquote expression)

Under a single quasiquote, ,expression escapes from the quote to include an evaluated expression whose result is inserted into the abbreviated list.

Under multiple quasiquotes, ,expression is really the literal ,expression, decrementing the quasiquote count by one for expression.

Normally, an unquote is written with ,, but it can also be written with unquote.

syntax
,@expression
syntax
(unquote-splicing expression)

Under a single quasiquote, ,@expression escapes from the quote to include an evaluated expression whose result is a list to splice into the abbreviated list.

Under multiple quasiquotes, a splicing unquote is like an unquote; that is, it decrements the quasiquote count by one.

Normally, a splicing unquote is written with ,, but it can also be written with unquote-splicing.

2.4 Common Syntaxes

The following syntaxes behave the same in the Beginner with List Abbreviations level as they did in the Beginning Student level.

syntax
(define (name variable variable ...) expression)

Defines a function named name. The expression is the body of the function. When the function is called, the values of the arguments are inserted into the body in place of the variables. The function returns the value of that new expression.

The function name’s cannot be the same as that of another function or variable.

syntax
(define name expression)

Defines a variable called name with the the value of expression. The variable name’s cannot be the same as that of another function or variable, and name itself must not appear in expression.

syntax
(define name (lambda (variable variable ...) expression))

An alternate way on defining functions. The name is the name of the function, which cannot be the same as that of another function or variable.

A lambda cannot be used outside of this alternate syntax.

syntax
(define-struct structure-name (field-name ...))

Defines a new structure called structure-name. The structure’s fields are named by the field-names. After the define-struct, the following new functions are available:

make-structure-name : takes a number of arguments equal to the number of fields in the structure, and creates a new instance of that structure.
structure-name-field-name : takes an instance of the structure and returns the value in the field named by field-name.
structure-name? : takes any value, and returns #true if the value is an instance of the structure.

The name of the new functions introduced by define-struct must not be the same as that of other functions or variables, otherwise define-struct reports an error.

syntax
(name expression expression ...)

Calls the function named name. The value of the call is the value of name’s body when every one of the function’s variables are replaced by the values of the corresponding expressions.

The function named name must defined before it can be called. The number of argument expressions must be the same as the number of arguments expected by the function.

syntax
(cond [question-expression answer-expression] ...)
(cond [question-expression answer-expression]
...
[else answer-expression])

Chooses a clause based on some condition. cond finds the first question-expression that evaluates to #true, then evaluates the corresponding answer-expression.

If none of the question-expressions evaluates to #true, cond’s value is the answer-expression of the else clause. If there is no else, cond reports an error. If the result of a question-expression is neither #true nor #false, cond also reports an error.

else cannot be used outside of cond.

syntax
(if question-expression
then-answer-expression
else-answer-expression)

When the value of the question-expression is #true, if evaluates the then-answer-expression. When the test is #false, if evaluates the else-answer-expression.

If the question-expression is neither #true nor #false, if reports an error.

syntax
(and expression expression expression ...)

Evaluates to #true if all the expressions are #true. If any expression is #false, the and expression evaluates to #false (and the expressions to the right of that expression are not evaluated.)

If any of the expressions evaluate to a value other than #true or #false, and reports an error.

syntax
(or expression expression expression ...)

Evaluates to #true as soon as one of the expressions is #true (and the expressions to the right of that expression are not evaluated.) If all of the expressions are #false, the or expression evaluates to #false.

If any of the expressions evaluate to a value other than #true or #false, or reports an error.

syntax
(check-expect expression expected-expression)

Checks that the first expression evaluates to the same value as the expected-expression.

(check-expect (fahrenheit->celsius 212) 100)
(check-expect (fahrenheit->celsius -40) -40)

(define (fahrenheit->celsius f)
(* 5/9 (- f 32)))

A check-expect expression must be placed at the top-level of a student program. Also it may show up anywhere in the program, including ahead of the tested function definition. By placing check-expects there, a programmer conveys to a future reader the intention behind the program with working examples, thus making it often superfluous to read the function definition proper.

It is an error for expr or expected-expr to produce an inexact number or a function value. As for inexact numbers, it is morally wrong to compare them for plain equality. Instead one tests whether they are both within a small interval; see check-within. As for functions (see Intermediate and up), it is provably impossible to compare functions.

syntax
(check-random expression expected-expression)

Checks that the first expression evaluates to the same value as the expected-expression.

The form supplies the same random-number generator to both parts. If both parts request random numbers from the same interval in the same order, they receive the same random numbers.

Here is a simple example of where check-random is useful:

(define WIDTH 100)
(define HEIGHT (* 2 WIDTH))

(define-struct player (name x y))
; A Player is (make-player String Nat Nat)

; String -> Player

(check-random (create-randomly-placed-player "David Van Horn")
(make-player "David Van Horn" (random WIDTH) (random HEIGHT)))

(define (create-randomly-placed-player name)
(make-player name (random WIDTH) (random HEIGHT)))

Note how random is called on the same numbers in the same order in both parts of check-random. If the two parts call random for different intervals, they are likely to fail:

; String -> Player

(check-random (create-randomly-placed-player "David Van Horn")
              (make-player "David Van Horn" (random WIDTH) (random HEIGHT)))

(define (create-randomly-placed-player name)
  (a-helper-function name (random HEIGHT)))

; Striing Number -> Player
(define (a-helper-function name height)
   (make-player name (random WIDTH) height))

Because the argument to a-helper-function is evaluated first, random is first called for the interval [0,HEIGHT) and then for [0,WIDTH), that is, in a different order than in the preceding check-random.

It is an error for expr or expected-expr to produce a function value or an inexact number; see note on check-expect for details.

syntax
(check-satisfied expression predicate)

Checks that the first expression satisfies the named predicate (function of one argument). Recall that “satisfies” means “the function produces #true for the given value.”

Here are simple examples for check-satisfied:

> (check-satisfied 1 odd?)
The test passed!

> (check-satisfied 1 even?)
Ran 1 test.
0 tests passed.
Check failures:
                     ┌───┐
        Actual value │ 1 │ does not satisfy even?.
                     └───┘
        at line 3, column 0

In general check-satisfied empowers program designers to use defined functions to formulate test suites:

; [cons Number [List-of Number]] -> Boolean
; a function for testing htdp-sort

(check-expect (sorted? (list 1 2 3)) #true)
(check-expect (sorted? (list 2 1 3)) #false)

(define (sorted? l)
  (cond
    [(empty? (rest l)) #true]
    [else (and (<= (first l) (second l)) (sorted? (rest l)))]))

; [List-of Number] -> [List-of Number]
; create a sorted version of the given list of numbers

(check-satisfied (htdp-sort (list 1 2 0 3)) sorted?)

(define (htdp-sort l)
  (cond
    [(empty? l) l]
    [else (insert (first l) (htdp-sort (rest l)))]))

; Number [List-of Number] -> [List-of Number]
; insert x into l at proper place
; assume l is arranged in ascending order
; the result is sorted in the same way
(define (insert x l)
  (cond
    [(empty? l) (list x)]
    [else (if (<= x (first l)) (cons x l) (cons (first l) (insert x (rest l))))]))

And yes, the results of htdp-sort satisfy the sorted? predicate:

> (check-satisfied (htdp-sort (list 1 2 0 3)) sorted?)

syntax
(check-within expression expected-expression delta)

Checks whether the value of the expression expression is structurally equal to the value produced by the expected-expression expression; every number in the first expression must be within delta of the corresponding number in the second expression.

(define-struct roots (x sqrt))
; RT is [List-of (make-roots Number Number)]

(define (root-of a)
  (make-roots a (sqrt a)))

(define (roots-table xs)
  (cond
    [(empty? xs) '()]
    [else (cons (root-of (first xs)) (roots-table (rest xs)))]))

Due to the presence of inexact numbers in nested data, check-within is the correct choice for testing, and the test succeeds if delta is reasonably large:

Example:

> (check-within (roots-table (list 1.0 2.0 3.0))
                (list
                  (make-roots 1.0 1.0)
                  (make-roots 2  1.414)
                  (make-roots 3  1.713))
                0.1)
The test passed!

In contrast, when delta is small, the test fails:

Example:

> (check-within (roots-table (list 2.0))
                (list
                  (make-roots 2  1.414))
                #i1e-5)
Ran 1 test.
0 tests passed.
Check failures:
                     ┌────────────────────────────────────────┐                                      ┌─────────────────────────┐
        Actual value │ '((make-roots 2.0 1.4142135623730951)) │ is not within 1e-5 of expected value │ '((make-roots 2 1.414)) │.
                     └────────────────────────────────────────┘                                      └─────────────────────────┘
        at line 5, column 0

It is an error for expressions or expected-expression to produce a function value; see note on check-expect for details.

If delta is not a number, check-within reports an error.

syntax
(check-error expression expected-error-message)
(check-error expression)

Checks that the expression reports an error, where the error messages matches the value of expected-error-message, if it is present.

Here is a typical beginner example that calls for a use of check-error:

(define sample-table
  '(("matthias" 10)
    ("matthew"  20)
    ("robby"    -1)
    ("shriram"  18)))

; [List-of [list String Number]] String -> Number
; determine the number associated with s in table

(define (lookup table s)
  (cond
    [(empty? table) (error (string-append s " not found"))]
    [else (if (string=? (first (first table)) s)
              (second (first table))
              (lookup (rest table)))]))

Consider the following two examples in this context:

Example:

> (check-expect (lookup sample-table "matthew") 20)
The test passed!

Example:

> (check-error (lookup sample-table "kathi") "kathi not found")
The test passed!

syntax
(check-member-of expression expression expression ...)

Checks that the value of the first expression is that of one of the following expressions.

; [List-of X] -> X
; pick a random element from the given list l
(define (pick-one l)
(list-ref l (random (length l))))

Example:

> (check-member-of (pick-one '("a" "b" "c")) "a" "b" "c")
The test passed!

It is an error for any of expressions to produce a function value; see note on check-expect for details.

syntax
(check-range expression low-expression high-expression)

Checks that the value of the first expression is a number in between the value of the low-expression and the high-expression, inclusive.

A check-range form is best used to delimit the possible results of functions that compute inexact numbers:

(define EPSILON 0.001)

; [Real -> Real] Real -> Real
; what is the slope of f at x?
(define (differentiate f x)
  (slope f (- x EPSILON) (+ x EPSILON)))

; [Real -> Real] Real Real -> Real
(define (slope-of f left right)
  (/ (- (f right) (f left))
     2 EPSILON))

(check-range (differentiate sin 0) 0.99 1.0)

It is an error for expression, low-expression, or high-expression to produce a function value; see note on check-expect for details.

syntax
(require string)

Makes the definitions of the module specified by string available in the current module (i.e., the current file), where string refers to a file relative to the current file.

The string is constrained in several ways to avoid problems with different path conventions on different platforms: a / is a directory separator, . always means the current directory, .. always means the parent directory, path elements can use only a through z (uppercase or lowercase), 0 through 9, -, _, and ., and the string cannot be empty or contain a leading or trailing /.

syntax
(require module-name)

Accesses a file in an installed library. The library name is an identifier with the same constraints as for a relative-path string (though without the quotes), with the additional constraint that it must not contain a ..

syntax
(require (lib string string ...))

Accesses a file in an installed library, making its definitions available in the current module (i.e., the current file). The first string names the library file, and the remaining strings name the collection (and sub-collection, and so on) where the file is installed. Each string is constrained in the same way as for the (require string) form.

syntax
(require (planet string (string string number number)))
syntax
(require (planet id))
syntax
(require (planet string))

Accesses a library that is distributed on the internet via the PLaneT server, making it definitions available in the current module (i.e., current file).

The full grammar for planet requires is given in Importing and Exporting: require and provide, but the best place to find examples of the syntax is on the the PLaneT server, in the description of a specific package.

2.5 Pre-defined Functions

The remaining subsections list those functions that are built into the programming language. All other functions are imported from a teachpack or must be defined in the program.

2.6 Numbers: Integers, Rationals, Reals, Complex, Exacts, Inexacts

procedure
(* x y z ...) → number
  x : number
  y : number
  z : number

Multiplies all numbers.

> (* 5 3)
15
> (* 5 3 2)
30

procedure
(+ x y z ...) → number
  x : number
  y : number
  z : number

Adds up all numbers.

> (+ 2/3 1/16)
35/48
> (+ 3 2 5 8)
18

procedure
(- x y ...) → number
x : number
y : number

Subtracts the second (and following) number(s) from the first ; negates the number if there is only one argument.

> (- 5)
-5
> (- 5 3)
2
> (- 5 3 1)
1

procedure
(/ x y z ...) → number
  x : number
  y : number
  z : number

Divides the first by the second (and all following) number(s).

> (/ 12 2)
6
> (/ 12 2 3)
2

procedure
(< x y z ...) → boolean?
  x : real
  y : real
  z : real

Compares (real) numbers for less-than.

> (< 42 2/5)
#false

procedure
(<= x y z ...) → boolean?
  x : real
  y : real
  z : real

Compares (real) numbers for less-than or equality.

> (<= 42 2/5)
#false

procedure
(= x y z ...) → boolean?
  x : number
  y : number
  z : number

Compares numbers for equality.

> (= 42 2/5)
#false

procedure
(> x y z ...) → boolean?
  x : real
  y : real
  z : real

Compares (real) numbers for greater-than.

> (> 42 2/5)
#true

procedure
(>= x y z ...) → boolean?
  x : real
  y : real
  z : real

Compares (real) numbers for greater-than or equality.

> (>= 42 42)
#true

procedure
(abs x) → real
x : real

Determines the absolute value of a real number.

> (abs -12)
12

procedure
(acos x) → number
x : number

Computes the arccosine (inverse of cos) of a number.

> (acos 0)
#i1.5707963267948966

procedure
(add1 x) → number
x : number

Increments the given number.

> (add1 2)
3

procedure
(angle x) → real
x : number

Extracts the angle from a complex number.

> (angle (make-polar 3 4))
#i-2.2831853071795867

procedure
(asin x) → number
x : number

Computes the arcsine (inverse of sin) of a number.

> (asin 0)
0

procedure
(atan x) → number
x : number

Computes the arctangent of the given number:

> (atan 0)
0
> (atan 0.5)
#i0.4636476090008061

Also comes in a two-argument version where (atan x y) computes (atan (/ x y)) but the signs of x and y determine the quadrant of the result and the result tends to be more accurate than that of the 1-argument version in borderline cases:

> (atan 3 4)
#i0.6435011087932844
> (atan -2 -1)
#i-2.0344439357957027

procedure
(ceiling x) → integer
x : real

Determines the closest integer (exact or inexact) above a real number. See round.

> (ceiling 12.3)
#i13.0

procedure
(complex? x) → boolean?
x : any/c

Determines whether some value is complex.

> (complex? 1-2i)
#true

procedure
(conjugate x) → number
x : number

Flips the sign of the imaginary part of a complex number.

> (conjugate 3+4i)
3-4i
> (conjugate -2-5i)
-2+5i
> (conjugate (make-polar 3 4))
#i-1.960930862590836+2.2704074859237844i

procedure
(cos x) → number
x : number

Computes the cosine of a number (radians).

> (cos pi)
#i-1.0

procedure
(cosh x) → number
x : number

Computes the hyperbolic cosine of a number.

> (cosh 10)
#i11013.232920103324

procedure
(current-seconds) → integer

Determines the current time in seconds elapsed (since a platform-specific starting date).

> (current-seconds)
1604929627

procedure
(denominator x) → integer
x : rational?

Computes the denominator of a rational.

> (denominator 2/3)
3

value
e : real

Euler’s number.

> e
#i2.718281828459045

procedure
(even? x) → boolean?
x : integer

Determines if some integer (exact or inexact) is even or not.

> (even? 2)
#true

procedure
(exact->inexact x) → number
x : number

Converts an exact number to an inexact one.

> (exact->inexact 12)
#i12.0

procedure
(exact? x) → boolean?
x : number

Determines whether some number is exact.

> (exact? (sqrt 2))
#false

procedure
(exp x) → number
x : number

Determines e raised to a number.

> (exp -2)
#i0.1353352832366127

procedure
(expt x y) → number
x : number
y : number

Computes the power of the first to the second number.

> (expt 16 1/2)
4
> (expt 3 -4)
1/81

procedure
(floor x) → integer
x : real

Determines the closest integer (exact or inexact) below a real number. See round.

> (floor 12.3)
#i12.0

procedure
(gcd x y ...) → integer
x : integer
y : integer

Determines the greatest common divisor of two integers (exact or inexact).

> (gcd 6 12 8)
2

procedure
(imag-part x) → real
x : number

Extracts the imaginary part from a complex number.

> (imag-part 3+4i)
4

procedure
(inexact->exact x) → number
x : number

Approximates an inexact number by an exact one.

> (inexact->exact 12.0)
12

procedure
(inexact? x) → boolean?
x : number

Determines whether some number is inexact.

> (inexact? 1-2i)
#false

procedure
(integer->char x) → char
x : exact-integer?

Looks up the character that corresponds to the given exact integer in the ASCII table (if any).

> (integer->char 42)
#\*

procedure
(integer-sqrt x) → complex
x : integer

Computes the integer or imaginary-integer square root of an integer.

> (integer-sqrt 11)
3
> (integer-sqrt -11)
0+3i

procedure
(integer? x) → boolean?
x : any/c

Determines whether some value is an integer (exact or inexact).

> (integer? (sqrt 2))
#false

procedure
(lcm x y ...) → integer
x : integer
y : integer

Determines the least common multiple of two integers (exact or inexact).

> (lcm 6 12 8)
24

procedure
(log x) → number
x : number

Determines the base-e logarithm of a number.

> (log 12)
#i2.4849066497880004

procedure
(magnitude x) → real
x : number

Determines the magnitude of a complex number.

> (magnitude (make-polar 3 4))
#i2.9999999999999996

procedure
(make-polar x y) → number
x : real
y : real

Creates a complex from a magnitude and angle.

> (make-polar 3 4)
#i-1.960930862590836-2.2704074859237844i

procedure
(make-rectangular x y) → number
x : real
y : real

Creates a complex from a real and an imaginary part.

> (make-rectangular 3 4)
3+4i

procedure
(max x y ...) → real
x : real
y : real

Determines the largest number—aka, the maximum.

> (max 3 2 8 7 2 9 0)
9

procedure
(min x y ...) → real
x : real
y : real

Determines the smallest number—aka, the minimum.

> (min 3 2 8 7 2 9 0)
0

procedure
(modulo x y) → integer
x : integer
y : integer

Finds the remainder of the division of the first number by the second:

> (modulo 9 2)
1
> (modulo 3 -4)
-1

procedure
(negative? x) → boolean?
x : real

Determines if some real number is strictly smaller than zero.

> (negative? -2)
#true

procedure
(number->string x) → string
x : number

Converts a number to a string.

> (number->string 42)
"42"

procedure
(number->string-digits x p) → string
x : number
p : posint

Converts a number x to a string with the specified number of digits.

> (number->string-digits 0.9 2)
"0.9"
> (number->string-digits pi 4)
"3.1416"

procedure
(number? n) → boolean?
n : any/c

Determines whether some value is a number:

> (number? "hello world")
#false
> (number? 42)
#true

procedure
(numerator x) → integer
x : rational?

Computes the numerator of a rational.

> (numerator 2/3)
2

procedure
(odd? x) → boolean?
x : integer

Determines if some integer (exact or inexact) is odd or not.

> (odd? 2)
#false

value
pi : real

The ratio of a circle’s circumference to its diameter.

> pi
#i3.141592653589793

procedure
(positive? x) → boolean?
x : real

Determines if some real number is strictly larger than zero.

> (positive? -2)
#false

procedure
(quotient x y) → integer
x : integer
y : integer

Divides the first integer—also called dividend—by the second—known as divisor—to obtain the quotient.

> (quotient 9 2)
4
> (quotient 3 4)
0

procedure
(random x) → natural
x : natural

Generates a random natural number less than some given exact natural.

> (random 42)
0

procedure
(rational? x) → boolean?
x : any/c

Determines whether some value is a rational number.

> (rational? 1)
#true
> (rational? -2.349)
#true
> (rational? #i1.23456789)
#true
> (rational? (sqrt -1))
#false
> (rational? pi)
#true
> (rational? e)
#true
> (rational? 1-2i)
#false

As the interactions show, the teaching languages considers many more numbers as rationals than expected. In particular, pi is a rational number because it is only a finite approximation to the mathematical π. Think of rational? as a suggestion to think of these numbers as fractions.

procedure
(real-part x) → real
x : number

Extracts the real part from a complex number.

> (real-part 3+4i)
3

procedure
(real? x) → boolean?
x : any/c

Determines whether some value is a real number.

> (real? 1-2i)
#false

procedure
(remainder x y) → integer
x : integer
y : integer

Determines the remainder of dividing the first by the second integer (exact or inexact).

> (remainder 9 2)
1
> (remainder 3 4)
3

procedure
(round x) → integer
x : real

Rounds a real number to an integer (rounds to even to break ties). See floor and ceiling.

> (round 12.3)
#i12.0

procedure
(sgn x) → (union 1 #i1.0 0 #i0.0 -1 #i-1.0)
x : real

Determines the sign of a real number.

> (sgn -12)
-1

procedure
(sin x) → number
x : number

Computes the sine of a number (radians).

> (sin pi)
#i1.2246467991473532e-16

procedure
(sinh x) → number
x : number

Computes the hyperbolic sine of a number.

> (sinh 10)
#i11013.232874703393

procedure
(sqr x) → number
x : number

Computes the square of a number.

> (sqr 8)
64

procedure
(sqrt x) → number
x : number

Computes the square root of a number.

> (sqrt 9)
3
> (sqrt 2)
#i1.4142135623730951

procedure
(sub1 x) → number
x : number

Decrements the given number.

> (sub1 2)
1

procedure
(tan x) → number
x : number

Computes the tangent of a number (radians).

> (tan pi)
#i-1.2246467991473532e-16

procedure
(zero? x) → boolean?
x : number

Determines if some number is zero or not.

> (zero? 2)
#false

2.7 Booleans

procedure
(boolean->string x) → string
x : boolean?

Produces a string for the given boolean

> (boolean->string #false)
"#false"
> (boolean->string #true)
"#true"

procedure
(boolean=? x y) → boolean?
x : boolean?
y : boolean?

Determines whether two booleans are equal.

> (boolean=? #true #false)
#false

procedure
(boolean? x) → boolean?
x : any/c

Determines whether some value is a boolean.

> (boolean? 42)
#false
> (boolean? #false)
#true

procedure
(false? x) → boolean?
x : any/c

Determines whether a value is false.

> (false? #false)
#true

procedure
(not x) → boolean?
x : boolean?

Negates a boolean value.

> (not #false)
#true

2.8 Symbols

procedure
(symbol->string x) → string
x : symbol

Converts a symbol to a string.

> (symbol->string 'c)
"c"

procedure
(symbol=? x y) → boolean?
x : symbol
y : symbol

Determines whether two symbols are equal.

> (symbol=? 'a 'b)
#false

procedure
(symbol? x) → boolean?
x : any/c

Determines whether some value is a symbol.

> (symbol? 'a)
#true

2.9 Lists

procedure
(append x y z ...) → list?
  x : list?
  y : list?
  z : list?

Creates a single list from several, by concatenation of the items.

> (append (cons 1 (cons 2 '())) (cons "a" (cons "b" empty)))
(list 1 2 "a" "b")

procedure
(assoc x l) → (union (listof any) #false)
x : any
l : (listof any)

Produces the first pair on l whose first is equal? to x; otherwise it produces #false.

> (assoc "hello" '(("world" 2) ("hello" 3) ("good" 0)))
(list "hello" 3)

procedure
(assq x l) → (union #false cons?)
x : any/c
l : list?

Determines whether some item is the first item of a pair in a list of pairs. (It compares the items with eq?.)

> a
(list (list 'a 22) (list 'b 8) (list 'c 70))
> (assq 'b a)
(list 'b 8)

procedure
(caaar x) → any/c
x : list?

LISP-style selector: (car (car (car x))).

> w
(list (list (list (list "bye") 3) #true) 42)
> (caaar w)
(list "bye")

procedure
(caadr x) → any/c
x : list?

LISP-style selector: (car (car (cdr x))).

> (caadr (cons 1 (cons (cons 'a '()) (cons (cons 'd '()) '()))))
'a

procedure
(caar x) → any/c
x : list?

LISP-style selector: (car (car x)).

> y
(list (list (list 1 2 3) #false "world"))
> (caar y)
(list 1 2 3)

procedure
(cadar x) → any/c
x : list?

LISP-style selector: (car (cdr (car x))).

> w
(list (list (list (list "bye") 3) #true) 42)
> (cadar w)
#true

procedure
(cadddr x) → any/c
x : list?

LISP-style selector: (car (cdr (cdr (cdr x)))).

> v
(list 1 2 3 4 5 6 7 8 9 'A)
> (cadddr v)
4

procedure
(caddr x) → any/c
x : list?

LISP-style selector: (car (cdr (cdr x))).

> x
(list 2 "hello" #true)
> (caddr x)
#true

procedure
(cadr x) → any/c
x : list?

LISP-style selector: (car (cdr x)).

> x
(list 2 "hello" #true)
> (cadr x)
"hello"

procedure
(car x) → any/c
x : cons?

Selects the first item of a non-empty list.

> x
(list 2 "hello" #true)
> (car x)
2

procedure
(cdaar x) → any/c
x : list?

LISP-style selector: (cdr (car (car x))).

> w
(list (list (list (list "bye") 3) #true) 42)
> (cdaar w)
(list 3)

procedure
(cdadr x) → any/c
x : list?

LISP-style selector: (cdr (car (cdr x))).

> (cdadr (list 1 (list 2 "a") 3))
(list "a")

procedure
(cdar x) → list?
x : list?

LISP-style selector: (cdr (car x)).

> y
(list (list (list 1 2 3) #false "world"))
> (cdar y)
(list #false "world")

procedure
(cddar x) → any/c
x : list?

LISP-style selector: (cdr (cdr (car x)))

> w
(list (list (list (list "bye") 3) #true) 42)
> (cddar w)
'()

procedure
(cdddr x) → any/c
x : list?

LISP-style selector: (cdr (cdr (cdr x))).

> v
(list 1 2 3 4 5 6 7 8 9 'A)
> (cdddr v)
(list 4 5 6 7 8 9 'A)

procedure
(cddr x) → list?
x : list?

LISP-style selector: (cdr (cdr x)).

> x
(list 2 "hello" #true)
> (cddr x)
(list #true)

procedure
(cdr x) → any/c
x : cons?

Selects the rest of a non-empty list.

> x
(list 2 "hello" #true)
> (cdr x)
(list "hello" #true)

procedure
(cons x y) → list?
x : any/x
y : list?

Constructs a list.

> (cons 1 '())
(cons 1 '())

procedure
(cons? x) → boolean?
x : any/c

Determines whether some value is a constructed list.

> (cons? (cons 1 '()))
#true
> (cons? 42)
#false

procedure
(eighth x) → any/c
x : list?

Selects the eighth item of a non-empty list.

> v
(list 1 2 3 4 5 6 7 8 9 'A)
> (eighth v)
8

procedure
(empty? x) → boolean?
x : any/c

Determines whether some value is the empty list.

> (empty? '())
#true
> (empty? 42)
#false

procedure
(fifth x) → any/c
x : list?

Selects the fifth item of a non-empty list.

> v
(list 1 2 3 4 5 6 7 8 9 'A)
> (fifth v)
5

procedure
(first x) → any/c
x : cons?

Selects the first item of a non-empty list.

> x
(list 2 "hello" #true)
> (first x)
2

procedure
(fourth x) → any/c
x : list?

Selects the fourth item of a non-empty list.

> v
(list 1 2 3 4 5 6 7 8 9 'A)
> (fourth v)
4

procedure
(length l) → natural-number?
l : list?

Evaluates the number of items on a list.

> x
(list 2 "hello" #true)
> (length x)
3

procedure
(list x ...) → list?
x : any/c

Constructs a list of its arguments.

> (list 1 2 3 4 5 6 7 8 9 0)
(cons 1 (cons 2 (cons 3 (cons 4 (cons 5 (cons 6 (cons 7 (cons 8 (cons 9 (cons 0 '()))))))))))

procedure
(list* x ... l) → list?
x : any/c
l : list?

Constructs a list by adding multiple items to a list.

> x
(list 2 "hello" #true)
> (list* 4 3 x)
(list 4 3 2 "hello" #true)

procedure
(list-ref x i) → any/c
x : list?
i : natural?

Extracts the indexed item from the list.

> v
(list 1 2 3 4 5 6 7 8 9 'A)
> (list-ref v 9)
'A

procedure
(list? x) → boolean?
x : any

Checks whether the given value is a list.

> (list? 42)
#false
> (list? '())
#true
> (list? (cons 1 (cons 2 '())))
#true

procedure
(make-list i x) → list?
i : natural-number
x : any/c

Constructs a list of i copies of x.

> (make-list 3 "hello")
(cons "hello" (cons "hello" (cons "hello" '())))

procedure
(member x l) → boolean?
x : any/c
l : list?

Determines whether some value is on the list (comparing values with equal?).

> x
(list 2 "hello" #true)
> (member "hello" x)
#true

procedure
(member? x l) → boolean?
x : any/c
l : list?

Determines whether some value is on the list (comparing values with equal?).

> x
(list 2 "hello" #true)
> (member? "hello" x)
#true

procedure
(memq x l) → boolean?
x : any/c
l : list?

Determines whether some value x is on some list l, using eq? to compare x with items on l.

> x
(list 2 "hello" #true)
> (memq (list (list 1 2 3)) x)
#false

procedure
(memq? x l) → boolean?
x : any/c
l : list?

Determines whether some value x is on some list l, using eq? to compare x with items on l.

> x
(list 2 "hello" #true)
> (memq? (list (list 1 2 3)) x)
#false

procedure
(memv x l) → (or/c #false list)
x : any/c
l : list?

Determines whether some value is on the list if so, it produces the suffix of the list that starts with x if not, it produces false. (It compares values with the eqv? predicate.)

> x
(list 2 "hello" #true)
> (memv (list (list 1 2 3)) x)
#false

value
null : list

Another name for the empty list

> null
'()

procedure
(null? x) → boolean?
x : any/c

Determines whether some value is the empty list.

> (null? '())
#true
> (null? 42)
#false

procedure
(range start end step) → list?
  start : number
  end : number
  step : number

Constructs a list of numbers by stepping from start to end.

> (range 0 10 2)
(cons 0 (cons 2 (cons 4 (cons 6 (cons 8 '())))))

procedure
(remove x l) → list?
x : any/c
l : list?

Constructs a list like the given one, with the first occurrence of the given item removed (comparing values with equal?).

> x
(list 2 "hello" #true)
> (remove "hello" x)
(list 2 #true)
> hello-2
(list 2 "hello" #true "hello")
> (remove "hello" hello-2)
(list 2 #true "hello")

procedure
(remove-all x l) → list?
x : any/c
l : list?

Constructs a list like the given one, with all occurrences of the given item removed (comparing values with equal?).

> x
(list 2 "hello" #true)
> (remove-all "hello" x)
(list 2 #true)
> hello-2
(list 2 "hello" #true "hello")
> (remove-all "hello" hello-2)
(list 2 #true)

procedure
(rest x) → any/c
x : cons?

Selects the rest of a non-empty list.

> x
(list 2 "hello" #true)
> (rest x)
(list "hello" #true)

procedure
(reverse l) → list
l : list?

Creates a reversed version of a list.

> x
(list 2 "hello" #true)
> (reverse x)
(list #true "hello" 2)

procedure
(second x) → any/c
x : list?

Selects the second item of a non-empty list.

> x
(list 2 "hello" #true)
> (second x)
"hello"

procedure
(seventh x) → any/c
x : list?

Selects the seventh item of a non-empty list.

> v
(list 1 2 3 4 5 6 7 8 9 'A)
> (seventh v)
7

procedure
(sixth x) → any/c
x : list?

Selects the sixth item of a non-empty list.

> v
(list 1 2 3 4 5 6 7 8 9 'A)
> (sixth v)
6

procedure
(third x) → any/c
x : list?

Selects the third item of a non-empty list.

> x
(list 2 "hello" #true)
> (third x)
#true

2.10 Posns

procedure
(make-posn x y) → posn
x : any/c
y : any/c

Constructs a posn from two arbitrary values.

> (make-posn 3 3)
(make-posn 3 3)
> (make-posn "hello" #true)
(make-posn "hello" #true)

procedure
(posn-x p) → any
p : posn

Extracts the x component of a posn.

> p
(make-posn 2 -3)
> (posn-x p)
2

procedure
(posn-y p) → any
p : posn

Extracts the y component of a posn.

> p
(make-posn 2 -3)
> (posn-y p)
-3

procedure
(posn? x) → boolean?
x : any/c

Determines if its input is a posn.

> q
(make-posn "bye" 2)
> (posn? q)
#true
> (posn? 42)
#false

2.11 Characters

procedure
(char->integer c) → integer
c : char

Looks up the number that corresponds to the given character in the ASCII table (if any).

> (char->integer #\a)
97
> (char->integer #\z)
122

procedure
(char-alphabetic? c) → boolean?
c : char

Determines whether a character represents an alphabetic character.

> (char-alphabetic? #\Q)
#true

procedure
(char-ci<=? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether the characters are ordered in an increasing and case-insensitive manner.

> (char-ci<=? #\b #\B)
#true
> (char<=? #\b #\B)
#false

procedure
(char-ci<? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether the characters are ordered in a strictly increasing and case-insensitive manner.

> (char-ci<? #\B #\c)
#true
> (char<? #\b #\B)
#false

procedure
(char-ci=? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether two characters are equal in a case-insensitive manner.

> (char-ci=? #\b #\B)
#true

procedure
(char-ci>=? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether the characters are sorted in a decreasing and case-insensitive manner.

> (char-ci>=? #\b #\C)
#false
> (char>=? #\b #\C)
#true

procedure
(char-ci>? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether the characters are sorted in a strictly decreasing and case-insensitive manner.

> (char-ci>? #\b #\B)
#false
> (char>? #\b #\B)
#true

procedure
(char-downcase c) → char
c : char

Produces the equivalent lower-case character.

> (char-downcase #\T)
#\t

procedure
(char-lower-case? c) → boolean?
c : char

Determines whether a character is a lower-case character.

> (char-lower-case? #\T)
#false

procedure
(char-numeric? c) → boolean?
c : char

Determines whether a character represents a digit.

> (char-numeric? #\9)
#true

procedure
(char-upcase c) → char
c : char

Produces the equivalent upper-case character.

> (char-upcase #\t)
#\T

procedure
(char-upper-case? c) → boolean?
c : char

Determines whether a character is an upper-case character.

> (char-upper-case? #\T)
#true

procedure
(char-whitespace? c) → boolean?
c : char

Determines whether a character represents space.

> (char-whitespace? #\tab)
#true

procedure
(char<=? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether the characters are ordered in an increasing manner.

> (char<=? #\a #\a #\b)
#true

procedure
(char<? x d e ...) → boolean?
  x : char
  d : char
  e : char

Determines whether the characters are ordered in a strictly increasing manner.

> (char<? #\a #\b #\c)
#true

procedure
(char=? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether the characters are equal.

> (char=? #\b #\a)
#false

procedure
(char>=? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether the characters are sorted in a decreasing manner.

> (char>=? #\b #\b #\a)
#true

procedure
(char>? c d e ...) → boolean?
  c : char
  d : char
  e : char

Determines whether the characters are sorted in a strictly decreasing manner.

> (char>? #\A #\z #\a)
#false

procedure
(char? x) → boolean?
x : any/c

Determines whether a value is a character.

> (char? "a")
#false
> (char? #\a)
#true

2.12 Strings

procedure
(explode s) → (listof string)
s : string

Translates a string into a list of 1-letter strings.

> (explode "cat")
(list "c" "a" "t")

procedure
(format f x ...) → string
f : string
x : any/c

Formats a string, possibly embedding values.

> (format "Dear Dr. ~a:" "Flatt")
"Dear Dr. Flatt:"
> (format "Dear Dr. ~s:" "Flatt")
"Dear Dr. \"Flatt\":"
> (format "the value of ~s is ~a" '(+ 1 1) (+ 1 1))
"the value of (+ 1 1) is 2"

procedure
(implode l) → string
l : list?

Concatenates the list of 1-letter strings into one string.

> (implode (cons "c" (cons "a" (cons "t" '()))))
"cat"

procedure
(int->string i) → string
i : integer

Converts an integer in [0,55295] or [57344 1114111] to a 1-letter string.

> (int->string 65)
"A"

procedure
(list->string l) → string
l : list?

Converts a s list of characters into a string.

> (list->string (cons #\c (cons #\a (cons #\t '()))))
"cat"

procedure
(make-string i c) → string
i : natural-number
c : char

Produces a string of length i from c.

> (make-string 3 #\d)
"ddd"

procedure
(replicate i s) → string
i : natural-number
s : string

Replicates s i times.

> (replicate 3 "h")
"hhh"

procedure
(string c ...) → string?
c : char

Builds a string of the given characters.

> (string #\d #\o #\g)
"dog"

procedure
(string->int s) → integer
s : string

Converts a 1-letter string to an integer in [0,55295] or [57344, 1114111].

> (string->int "a")
97

procedure
(string->list s) → (listof char)
s : string

Converts a string into a list of characters.

> (string->list "hello")
(list #\h #\e #\l #\l #\o)

procedure
(string->number s) → (union number #false)
s : string

Converts a string into a number, produce false if impossible.

> (string->number "-2.03")
#i-2.03
> (string->number "1-2i")
1-2i

procedure
(string->symbol s) → symbol
s : string

Converts a string into a symbol.

> (string->symbol "hello")
'hello

procedure
(string-alphabetic? s) → boolean?
s : string

Determines whether all ’letters’ in the string are alphabetic.

> (string-alphabetic? "123")
#false
> (string-alphabetic? "cat")
#true

procedure
(string-append s t z ...) → string
  s : string
  t : string
  z : string

Concatenates the characters of several strings.

> (string-append "hello" " " "world" " " "good bye")
"hello world good bye"

procedure
(string-ci<=? s t) → boolean?
s : string
t : string

Determines whether the strings are ordered in a lexicographically increasing and case-insensitive manner.

> (string-ci<=? "hello" "WORLD")
#true

procedure
(string-ci<? s t) → boolean?
s : string
t : string

Determines whether the strings are ordered in a lexicographically strictly increasing and case-insensitive manner.

> (string-ci<? "hello" "WORLD")
#true

procedure
(string-ci=? s t) → boolean?
s : string
t : string

Determines whether all strings are equal, character for character, regardless of case.

> (string-ci=? "hello" "HellO")
#true

procedure
(string-ci>=? s t) → boolean?
s : string
t : string

Determines whether the strings are ordered in a lexicographically decreasing and case-insensitive manner.

> (string-ci>? "WORLD" "hello")
#true

procedure
(string-ci>? s t) → boolean?
s : string
t : string

Determines whether the strings are ordered in a lexicographically strictly decreasing and case-insensitive manner.

> (string-ci>? "WORLD" "hello")
#true

procedure
(string-contains-ci? s t) → boolean?
s : string
t : string

Determines whether the first string appears in the second one without regard to the case of the letters.

> (string-contains-ci? "At" "caT")
#true

procedure
(string-contains? s t) → boolean?
s : string
t : string

Determines whether the first string appears literally in the second one.

> (string-contains? "at" "cat")
#true

procedure
(string-copy s) → string
s : string

Copies a string.

> (string-copy "hello")
"hello"

procedure
(string-downcase s) → string
s : string

Produces a string like the given one with all ’letters’ as lower case.

> (string-downcase "CAT")
"cat"
> (string-downcase "cAt")
"cat"

procedure
(string-ith s i) → 1string?
s : string
i : natural-number

Extracts the ith 1-letter substring from s.

> (string-ith "hello world" 1)
"e"

procedure
(string-length s) → nat
s : string

Determines the length of a string.

> (string-length "hello world")
11

procedure
(string-lower-case? s) → boolean?
s : string

Determines whether all ’letters’ in the string are lower case.

> (string-lower-case? "CAT")
#false

procedure
(string-numeric? s) → boolean?
s : string

Determines whether all ’letters’ in the string are numeric.

> (string-numeric? "123")
#true
> (string-numeric? "1-2i")
#false

procedure
(string-ref s i) → char
s : string
i : natural-number

Extracts the ith character from s.

> (string-ref "cat" 2)
#\t

procedure
(string-upcase s) → string
s : string

Produces a string like the given one with all ’letters’ as upper case.

> (string-upcase "cat")
"CAT"
> (string-upcase "cAt")
"CAT"

procedure
(string-upper-case? s) → boolean?
s : string

Determines whether all ’letters’ in the string are upper case.

> (string-upper-case? "CAT")
#true

procedure
(string-whitespace? s) → boolean?
s : string

Determines whether all ’letters’ in the string are white space.

> (string-whitespace? (string-append " " (string #\tab #\newline #\return)))
#true

procedure
(string<=? s t) → boolean?
s : string
t : string

Determines whether the strings are ordered in a lexicographically increasing manner.

> (string<=? "hello" "hello" "world")
string<=?:expects only 2 arguments, found 3

procedure
(string<? s t) → boolean?
s : string
t : string

Determines whether the strings are ordered in a lexicographically strictly increasing manner.

> (string<? "hello" "world")
#true

procedure
(string=? s t) → boolean?
s : string
t : string

Determines whether all strings are equal, character for character.

> (string=? "hello" "world")
#false
> (string=? "bye" "bye")
#true

procedure
(string>=? s t) → boolean?
s : string
t : string

Determines whether the strings are ordered in a lexicographically decreasing manner.

> (string>=? "world" "hello")
#true

procedure
(string>? s t) → boolean?
s : string
t : string

Determines whether the strings are ordered in a lexicographically strictly decreasing manner.

> (string>? "world" "hello")
#true

procedure
(string? x) → boolean?
x : any/c

Determines whether a value is a string.

> (string? "hello world")
#true
> (string? 42)
#false

procedure
(substring s i j) → string
  s : string
  i : natural-number
  j : natural-number

Extracts the substring starting at i up to j (or the end if j is not provided).

> (substring "hello world" 1 5)
"ello"
> (substring "hello world" 4)
"o world"

2.13 Images

procedure
(image=? i j) → boolean?
i : image
j : image

Determines whether two images are equal.

> c1
> (image=? (circle 5 "solid" "green") c1)
#false
> (image=? (circle 10 "solid" "green") c1)
#true

procedure
(image? x) → boolean?
x : any/c

Determines whether a value is an image.

> c1
> (image? c1)
#true

2.14 Misc

procedure
(=~ x y eps) → boolean?
  x : number
  y : number
  eps : non-negative-real

Checks whether x and y are within eps of either other.

> (=~ 1.01 1.0 0.1)
#true
> (=~ 1.01 1.5 0.1)
#false

value
eof : eof-object?

A value that represents the end of a file:

> eof
#<eof>

procedure
(eof-object? x) → boolean?
x : any/c

Determines whether some value is the end-of-file value.

> (eof-object? eof)
#true
> (eof-object? 42)
#false

procedure
(eq? x y) → boolean?
x : any/c
y : any/c

Determines whether two values are equivalent from the computer’s perspective (intensional).

> (eq? (cons 1 '()) (cons 1 '()))
#false
> one
(list 1)
> (eq? one one)
#true

procedure
(equal? x y) → boolean?
x : any/c
y : any/c

Determines whether two values are structurally equal where basic values are compared with the eqv? predicate.

> (equal? (make-posn 1 2) (make-posn (- 2 1) (+ 1 1)))
#true

procedure
(equal~? x y z) → boolean?
  x : any/c
  y : any/c
  z : non-negative-real

Compares x and y like equal? but uses =~ in the case of numbers.

> (equal~? (make-posn 1.01 1.0) (make-posn 1.01 0.99) 0.2)
#true

procedure
(eqv? x y) → boolean?
x : any/c
y : any/c

Determines whether two values are equivalent from the perspective of all functions that can be applied to it (extensional).

> (eqv? (cons 1 '()) (cons 1 '()))
#false
> one
(list 1)
> (eqv? one one)
#true

procedure
(error x ...) → void?
x : any/c

Signals an error, combining the given values into an error message. If any of the values’ printed representations is too long, it is truncated and “...” is put into the string. If the first value is a symbol, it is suffixed with a colon and the result pre-pended on to the error message.

> zero
0
> (if (= zero 0) (error "can't divide by 0") (/ 1 zero))
can't divide by 0

procedure
(exit) → void

Evaluating (exit) terminates the running program.

procedure
(identity x) → any
x : any/c

Returns x.

> (identity 42)
42
> (identity c1)
> (identity "hello")
"hello"

procedure
(struct? x) → boolean?
x : any/c

Determines whether some value is a structure.

> (struct? (make-posn 1 2))
#true
> (struct? 43)
#false

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